Effects of long-term fertilizer phosphorus application on soil and crop phosphorus and cadmium contents

The effects of four rates of fertilizer phosphorus (P) application (0, 9·8, 19·6 and 39·2 kg P/ha per year) on soil and crop P and cadmium (Cd) contents were measured in a field trial begun in 1968 and cropped each year with barley in south west England. In 1996, available and total soil P and Cd were measured in seven soil layers (0–20, 20–25, 25–30, 30–35, 35–40, 40–45 and 45–50 cm). Offtake of P in the crop was measured, or could be estimated, throughout the trial period. There was a linear relationship between P balance (total applied − total offtake) and P application rate with a balance of zero at a rate equivalent to 17 kg P/ha per year. The rate of P required for the economically optimum grain output was equivalent to 30 kg P/ha per year. No evidence was found for available P enrichment of soil layers below 25 cm. There was no evidence of Cd enrichment of either soil or crop after 29 years of P applications.